Treating sewage or like sludge

ABSTRACT

There is disclosed a process for treating sewage sludge or similar organic sludge in which dewatered sludge cake and an alkaline admixture are mixed and then dried under pasteurisation conditions utilising heat from the exothermic reaction with the alkali, in which drying is effected by extraction of moisture evaporated from the mixture by the exothermic heat.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to treating sewage sludge or similarorganic sludge, such as that obtained from industrial sites such aswaste water treatment plants.

[0003] 2. Description of the Prior Art

[0004] EP 0 283 153 B1 discloses a method of treating wastewater sludgeto provide a fertiliser for agricultural lands which can be applieddirectly to the land. The method involves mixing the sludge withalkaline material of specified fineness to raise the pH of the mixtureto at least 12 for at least a day and to effect pasteurisation, and thendrying the mixture. Drying is done either a) by aeration and maintainingthe pH above 12 for at least seven days until the solids levels reachand maintain a minimum of 65% bulk solids, or b) by aeration and heatingto a temperature of at least 50° C. and so that the solids levels reachand maintain a minimum of 50% solids. In b) the temperature should notbe so high as to destroy all non-pathogenic organisms. The elevation ofthe pH and the drying are effected so as to reduce odour and undesirableviruses, bacteria, parasites and vector (e.g. flies) attraction to thesludge and prevent significant pathogen regrowth while not eliminatingbeneficial non-pathogenic microorganisms. At least some of thetemperature rise is due to the exothermic reaction with the alkali. Theproduct is allowed to air cure for about 10 days after achieving thedesired solids content. Drying and curing may be effected by windrowing,turnover or other forced air methods, and curing or aeration time isdependent on the aeration procedure and other factors and clearly needto be determined, for any particular circumstances, experimentally tosee that the required end point has been reached. This involves takingsamples to measure the solids content, testing for:

[0005] animal viruses—less than one plaque forming unit per 100 ml

[0006] salmonella bacteria—less than three colony forming units per 100ml

[0007] parasites—less than one viable egg per 100 ml

[0008] significant regrowth of the pathogenic microorganisms—thereshould be none

[0009] and assessing the reduction of odour to a level tolerable in aclosed room and that this reduction is maintained indefinitely under anyclimatic conditions, and whether flies are less attracted to theproduct. At the same time, the presence of at least some beneficialnon-pathogenic microorganisms must be established.

[0010] If method b) is used, the minimum recommended time formaintaining the temperature at least 50° C. (but not so high as todestroy all non-pathogenic microorganisms) is at least 12 hours and theheat treated alkaline stabilised dewatered sludge cake is then air dried(while the pH remains above 12 for at least 3 days) through intermittentturning of windrows until a minimum of 50% solids content is achieved.

[0011] GB 2 276 876 A, which refers to EP 0 283 153, describes treatmentof sewage sludge having a solids content of at least 15% by weight withan alkaline material containing free lime, and storing and/or dryingand/or composting the resultant mixture, adding sufficient lime to themixture to achieve a pH of at least 10. Higher pH values, even higherthan 12, are not ruled out, but it is suggested that excellent reductionin pathogens can be achieved at pH levels below 12 with reducedevolution of ammonia, which renders the treated sludge and itssurroundings less unpleasant and means that the treated material retainsmore nitrogen increasing its value as a fertiliser.

[0012] The method of GB 2 276 876 involves dewatering the sludge, e.g.in a press, and measuring it using a weigh hopper into a mixer intowhich is delivered alkaline material in the appropriate amount, themixed material being delivered to a skip to deliver it to a windrowingarea where it is turned periodically, normally for a period up to sevendays. It is suggested also that provided the amount of calcium oxidethat has been introduced is sufficient to raise the pH to a valuegreater than 12, and sufficient, indeed, to ensure tat it stays above 12for at least 2 hours, then it is possible to store the mix for not lessthan two hours, but then apply it directly to the land. However, GB 2276 876 notes that such a product is not sterile.

[0013] It is apparent that the methods described in these twospecifications are labour intensive and to a large extent dependent upona subjective judgement (the assessment of odour in a closed room, forexample) or upon results of biological tests which themselves could takedays or weeks before you have a result.

[0014] It is necessary, for effective treatment, to mix the alkali withdewatered sludge, but sludge, nonetheless that comprises a substantialquantity of water, as it is only in the presence of water that thedesired exothermic chemical reaction takes place. The end product—as isclear from the specification discussed—is required to have less watercontent than is appropriate during the heat-development stage. It is anessential part of the process, then, that the product be eventuallydried, and this is the reason for treating it by windrowing. Windrowing,especially aeration by turning over windrows, is not only labourintensive and floor space intensive, it is a slow method of drying.

[0015] There are further problems and disadvantages with prior artmethods such as those disclosed in EP 0 283 153 and GB 2 276 876. Forexample, it is generally accepted wisdom within the art that the step ofpasteurisation should take at least a day: the reader is directed to EP0 283 153 B1 for a detailed discussion of this point.

[0016] International Publication No. WO 98/29348 discloses a differentapproach to the treatment of sewage sludge to those disclosed in EP 0283 153 and GB 2 276 876. In WO 98/29348, drying is effected solely byheating caused by exothermic reaction with an alkaline admixture. Thus,the approach of WO 98/29348 has the considerable advantage that the timeand space consuming step of aeration is not required. This results inimprovements in processing speeds—near continuous or continuousoperation is possible. Furthermore, there are substantial reductions inthe amount of floor space required for a processing plant which employsthe method of WO 98/29348.

[0017] However, it would be highly desirable to provide furtherreductions in the processing time required to treat sewage sludge,whilst maintaining or improving the quality of the final product, sincethroughput can then be increased.

SUMMARY OF THE INVENTION

[0018] The present invention overcomes the aforementioned problems anddisadvantages, and overturns the received wisdom in the art regardingthe nature of the pasteurisation step. Further the present inventiondoes not utilise time and space aeration techniques.

[0019] According to the invention there is provided a process fortreating sewage sludge or similar organic sludge in which dewateredsludge cake and an alkaline admixture are mixed and then dried underpasteurisation conditions utilising heat from the exothermic reactionwith the alkali; in which drying is effected by extraction of moistureevaporated from the mixture by the exothermic heat;

[0020] characterised in that the pasteurisation conditions comprisemaintaining a pasteurising temperature for less than 10 hours at a pH ofat least 12, the pH of the mixture being maintained at 12 or above forless than a day.

[0021] Surprisingly, it has been found that such treatment, hithertoregarded as too short, can effect pasteurisation of the sludge. Inparticular, E. coli 157 and Ascaris species can be completely destroyed,or killed to an extent to which only a few organisms or eggs survive.Thus, the processing time required in order to treat the sludge isreduced.

[0022] Preferably, the pasteurising temperature is maintained for lessthan 8 hours, more preferably less than 6 hours, most preferably forabout 4 hours. The time during which the mixture is at 12 or above is,necessarily, longer than the time spent at the pasteurising temperatureHowever, it is desirable that the mixture is at 12 or above for as shorta period of time as possible.

[0023] The drying may be effected to dry the product to at least 50%,preferably at least 55%, dry solids content.

[0024] The drying may be effected to dry the product to between 50 and65%, preferably between 55 and 60% dry solids content. The dryingproduct may pass two or more times through a drying hopper. Drying maybe assisted by an air flow, which may be fan generated. Gases given offduring drying may be passed through an ammonia scrubber.

[0025] It is important that the product is relatively dry, sincemicroorganisms can survive and regenerate in a wet environment, whereasthey cannot do so in an aggressive dry environment. The combination ofhigh pH and high dry solids—in excess of 50% offers long term stabilityof the product without pathogen regrowth.

[0026] The drying may be effected in a first in first out (FIFO) hopper.

[0027] Drying may be effected in a cycling operation until apredetermined moisture content is detected, then the dried material isdischarged.

[0028] The dried material may be tested for moisture (or dry solidscontent) by a sensor the output of which determines whether the productis recycled or delivered to another location, e.g. a stockpile. Thedried mixture may be tested by an infra-red moisture measuringtransducer.

[0029] At the beginning of the process, the dewatered sludge may bemeasured for water content and the measure used to control the alkalineadmixture—the dewatered sludge may pass beneath an infra-red moisturemeasuring transducer.

[0030] The dewatered sludge cake may be fed to a storage bin from whereit is delivered to a mixing arrangement with the alkaline admixture. Thestorage bin may have a level sensor controlling the supply of sludgecake to the storage bin and/or delivery of sludge cake to the mixingarrangement. The delivery of sludge cake to tie mixing arrangement maybe measured and the measurement used to control the supply of alkalineadmixture to the mixing arrangement. Delivery of sludge cake to themixing arrangement may be effected by a weigh belt.

[0031] The moisture content of the mixed dewatered sludge cake andalkaline admixture may be measured and the measurement may be used tocontrol the addition of said admixture to said sludge cake.

[0032] The moisture content of the mixed dewatered sludge cake andalkaline admixture may be measured before drying and the measurementused to control the drying process.

[0033] Dewatered sludge cake may be fed directly from a dewateringarrangement such for example comprising a belt press arrangement.

[0034] The belt press arrangement may comprise primary and secondaryhigh pressure belt press arrangements.

[0035] The output from the infra-red moisture measuring transducer maybe used to control sludge cake throughout via adjustment of theoperation of the belt process arrangement.

[0036] The process may be controlled by a microprocessor.

[0037] Feedback loops may be used to control operational parameters(such as feed rates, alkaline admixture mixing ratios andpolyelectrolyte dosing). The feedback loops may comprise proportionalintegral differential loops.

[0038] The steps of mixing, filing, drying and discharging the productmay be performed within 24 hours. The step of filling comprises fillinga pasteursation unit such as a FIFO hopper. The mixing and filling maybe performed in less than 8 hours, drying in less than 5 hours anddischarging in less than 3 hours.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Processes in accordance with the invention will now be describedwith reference to the accompanying drawings, in which:

[0040]FIG. 1 is a plan view of a sludge cake-alkali mixing apparatus;and

[0041]FIG. 2 is a schematic diagram of apparatus for introducing sludgecake to the mixing apparatus;

DETAILED DESCRIPTION OF THE INVENTION

[0042] The invention comprises a process for treating sewage sludge andsimilar organic sludge in which dewatered sludge cake and an alkalineadmixture are mixed and then dried under pasteurisation conditionsutilising heat from the exothermic reaction with the alkali, in whichdrying is effected by extraction of moisture evaporated from the mixtureby the exothermic heat. Apparatus for performing the process aredepicted in FIGS. 1 and 2.

[0043] Pasteurisation and drying, by aeration, is effected in a first infirst out (FIFO) hopper 10 such as a 100% live capacity Samson hopperfined with full width discharger 10 a.

[0044] The drying is effected in a cycling operation until apredetermined moisture content is detected, then the dried material isdischarged via outloading belt conveyor 12, and take away belt conveyor26.

[0045] The drying is effected to dry the product to between 50 and 65%,preferably between 55 and 60%, dry solids content. The drying productpasses two or more times through the drying hopper 10, carried by aninclined belt conveyor 14, reversing belt conveyor 16, screw feederdischarger 18 and recycling belt conveyor 20. Drying is assisted by anair flow which may be fan generated by an extraction fan (not shown).The air flow is distributed across the cross section and length of thehopper 10.

[0046] Gases given off during drying are passed through an ammoniascrubber 24. Thus discharges to atmosphere are treated, and do notpresent a health risk either to the process operators or to the generalpublic. Residual material from the ammonia scrubber 24 may be mixed withthe final dried product to eliminate a waste stream and to enhance thenutrient value of the final product.

[0047] The product is held for a maximum of 10 hours at pasteurisationtemperature, which is monitored by temperature probes (not shown).Typically, the pasteurisation temperature is in the range 50-55° C.,although the use of higher temperatures still is not ruled out.

[0048] Surprisingly, it has been found that such a short heat treatmentis sufficient to effect pasteurisation of the sludge. It is possible tohold the product at pasteurisation temperature for less than 8 hours,even less than 6 hours. In fact, maintenance of the pasteurisationtemperature for about 4 hours has been found to be effective. It is notyet known if shorter heat treatment might be effective. It isanticipated that a treatment time of 2 hours, for example, would proveto be insufficiently long, but this has not yet been confirmed. For thisreason, a numerical lower time limit is not provided herein (in anyevent, such a lower time limit would likely depend on the specificapplication and nature of the sludge). However, the skilled person wouldhave no difficulty in ascertaining, if he/she so wished, such a lowerlimit.

[0049] The present invention can effect complete or near completedestruction of harmful pathogens, in particular E.coli 157 and Ascarisspecies. Ascaris species are regarded as being particularly difficult todestroy—see, for example, EP 0 283 153.

[0050] The pH of the mixture is maintained for less than a day,preferably less than 12 hours. This can be ensured by providing analkaline admixture which comprises (CaO) and one or more furtheraddivitives, such as fly ash. The amount of lime used is enough—but justenough—to obtain the required rise in temperature. The fly ash (which isat a pH of ca. 9.5) provides granularity and bulk in the final product.

[0051] The dried material is tested for moisture (or dry solids content)by a sensor (not shown), the output of which determines whether theproduct is recycled or delivered to another location, e.g. a stockpile,via the reversing belt conveyor 16 and take away belt conveyor 26. Thedried material may be tested by an infra-red moisture measuringtransducer. The process achieves total exhaustion of free calcium oxidewith the water in the drying product. Additionally, when the product isready to be discharged, a reduction in temperature to around 25° C. isachieved.

[0052]FIG. 2 depicts apparatus used at earlier stages of the process. Atthe beginning of the process, the dewatered sludge is measured for watercontent and the measurement used to control the alkaline admixture. Thisis achieved by passing the dewatered sludge beneath an infra-redmoisture measuring transducer 30, and subsequently controlling certainoperations in a manner described more fully below.

[0053] The dewatered sludge cake is fed—via a belt conveyor 32—to astorage bin 34 from where it is delivered to a mixing arrangement 36with the alkaline admixture. The storage bin 34 has an ultrasonic levelsensor 38 controlling the supply of sludge cake to the storage bin 34and/or delivery of sludge cake to the mixing arrangement 36. Thedelivery of sludge cake to the mixing arrangement 36 is measured and themeasurement used to control the supply of alkaline admixture to themixing arrangement 36. Delivery of sludge cake (exiting the storage bin34 via a sliding frame discharger 34 a and screw feeder 34 b) to themixing arrangement is effected by a weigh belt feeder 40, which permitsthe measurement of sludge cake delivery by monitoring the weight ofsludge cake fed.

[0054] The mixing arrangement 36 provides complete and intimate contactbetween the sludge cake and the alkaline admixtures without breaking thestructure of the sludge cake (which would result in a “plasticised”,paste-like material). Alkaine admixture is supplied from two silos 42,44 via screw conveyors 46, 48. Further silos may be employed.Additionally, a lime silo 52 is provided. The purpose of the lime silo52 is to provide small, controlled additions of CaO or a material havinga high free CaO content (for example, lime kiln dust) in the event thatthe other alkaline admixtures do not achieve the required temperaturerise and hold criteria. The mixed dewatered sludge cake and alkalineadmixture is transferred via a swivel belt conveyor 49 to the inclinedbelt conveyor 14 which supplies the hopper 10. The moisture content ofthe mixed sludge cake and alkaline admixture is measured before dryingand the measurement used to control the drying process. The measurementis made by an infra-red moisture measuring transducer 50, the output ofwhich may also be used to adjust the feed rates of the alkalineadmixture. This is used to achieve the desired mixed product moisturesetting, typically 50%,

[0055] The dewatered sludge cake may be fed directly from a dewateringarrangement, which comprises a belt press arrangement (not shown). Thebelt press arrangement involves both primary and secondary belt pressingoperations. Such an arrangement results in a number of advantages, suchas an increase in dry solids content from a norm of ca. 25% to a norm ofca. 33 to 38%. Furthermore, substantial reductions in sludge caketonnage feed, alkaline admixture, mixed product and overall plant sizeare found to occur.

[0056] The process is controlled by an industrial process controller(PLC) with PC based data acquisition. The PLC provides automatic controlof material handling, mixing, and the pasteurisation/aeration process.The control and automation system incorporates proportional integraldifferential (PID) feedback loop controls to optimise process economy(e.g. admixture consumption and polyclectrolyte useage duringdewatering) within the constraints of the required operating parameters.The use of other feedback systems is within the scope of the invention.Inputs from the moisture sensors and tachometers fitted to the feedscrews (which provide indications of speeds and thus feed rates) areused in the control loops, although the master control is provided bythe weight indication given by the weigh belt feeder 40. Variables suchas polyelectrolyte dosing, and belt process speed and throughput arealso under microprocessor control. The output from infra-red moisturemeasuring transducer 30 may be used to control sludge cake throughputvia adjustment of the operation of belt press arrangement, in particularto optimise dry solids output with respect to polyelectrolyte additions.

[0057] The system software provides both visual and hard copy on line,reporting on all of the process steps described above. Daily productionreports, heat pulse temperature graphs, and sludge, mixed and finalproduct dry solids reports can be made available. Additionally, a “helpline” service from the operating site to Head Office is provided via amodem link, with a remote management reporting facility.

[0058] It is possible to provide essentially continuous treatment bycareful control of treatment times and operating conditions. With asingle pasteurisation unit (such as the FIFO hopper 10) it is desirableto complete the entire process of mixing and filling, temperature riseand hold, recycling (comprising aeration and drying) and dischargewithin a 24 hour cycle. The major difficulty with the method of WO98/29348 is that the pasteurisation temperature (typically 52 to 60° C.)should be maintained for at least 12 hours, with another two hours or soadditionally being required to attain this temperature, The presentinvention provides a significant improvement in this regard, since thepasteurisation temperature is maintained for a substantially reducedperiod of time.

[0059] The key process considerations required to achieve the 24 hourcycle are:

[0060] (a) Mixing and filling operations must be rapid. This involvessuch considerations as increased dewatering machine capacities andoperating regimes, sludge buffer storage capacity increase and resultantmixing machine throughput requirement.

[0061] (b) The recycle aeration and drying phase must be also rapid andefficient as perhaps only one complete recycle can be achieved. Specialconsiderations during this phase are product discharge and recycle ratesand ventilation requirements.

[0062] (c) Finished product discharge to stockpile must also be at ahigh rate.

[0063] In order also to achieve (a), (b) and (c) above, total variablespeed control of the dewatering machines, mixer drivers and the massdischarger is required. Such is provided using the PLC system describedabove.

[0064] If a twin pasteuriser arrangement is employed, a continuousoperational cycle can be employed whilst enjoying 100% back-up capacity.In such an arrangement, an extra hopper might be provided to the setupshown in FIG. 1, swivel belt 49 alternating between the two hoppers.Alternatively, both pasteurisers can be used in tandem.

[0065] The advantages of the continuous treatment include reduced plantrequirements, a reduced plant footprint, increased processing throughputand reduced operating labour and power costs per tonne of sludgeprocessed, since most of the plant operations are carried out when thesite is unmanned.

[0066] Modifications to the process described above will be apparent tothose skilled in the art. For example, more admixture silos or an extrahopper for pasteurisation/drying might be provided.

1. A process for treating sewage sludge or similar organic sludge inwhich dewatered sludge cake and an alkaline admixture are mixed and thendried under pasteurisation conditions utilising heat from the exothermicreaction with the alkali, in which drying is effected by extraction ofmoisture evaporated from the mixture by the exothermic heat;characterised in that the pasteurisation conditions comprise maintaininga pasteurising temperature for less than 10 hours at a pH of at least12, the pH of the mixture being maintained at 12 or above for less thana day.
 2. A process according to claim 1 , in which the pasteurisingtemperature is maintained for less than 8 hours, preferably less than 6hours, more preferably for about 4 hours.
 3. A process according toclaim 2 , in which drying is effected to dry the product to at least50%, preferably at least 55%, dry solids content.
 4. A process accordingto claim 3 , in which the drying is effected to dry the product tobetween 50 and 65% dry solids content.
 5. A process according to claim 4, in which the drying is effected to dry the product to between 55 and60% dry solids content.
 6. A process according to claim 1 , in whichdrying is effected in a FIFO hopper.
 7. A process according to claim 1 ,in which drying is effected in a cycling operation until a predeterminedmoisture content is detected, then the dried material is discharged. 8.A process according to claim 1 , in which drying product passes two ormore times through a drying hopper.
 9. A process according to claim 1 ,in which the steps of mixing, filling, drying and discharging theproduct are performed within 24 hours.
 10. A process according to claim9 in which the mixing and filling is performed in less than 8 hours. 11.A process according to claim 9 , in which the drying is performed inless than 5 hours.
 12. A process according to claim 9 , in which thedischarging is performed in less than 3 hours.